The Victory Banner symbol represents how Buddha won over the demon Mara. This demon, in Buddhism, is synonymous to passion, lust and pride. The Banner of Victory is used to remind people that one must win over their own pride, lust and passions to be able to reach enlightenment.
Similarly the gods elected to place a banner of victory on the summit of Mt Meru, to honour the Buddha as the ‘Conqueror’ who defeated the armies of Mara